For reasons we can no longer fully understand, the original understanding of the Cross as a chi-rho (i.e. the the Greek name of the letters CR) were systematically replaced with 'ordinary' depictions of T-shaped crosses. This process only starts in the latter part of the fourth century. Indeed before that time all representations of the Cross looked like this:
Indeed it isn't just artistic representations of the crucifixion which were affected. The ancient texts of Christianity were also altered.
One of the earliest documents from Clement of Alexandria's Egyptian church is the Epistle of Barnabas, a letter purportedly written by a companion of the disciples. Our surviving copies of that letter makes it seem as if Barnabas promoted the idea of a T-shaped cross through a mystical interpretation of a passage in Book of Genesis. Barnabas took a deep interest in the story of how Abraham charged into battle with 318 men. Jews have always been interested in this story because it supports their claim that there are 'codes' or 'ciphers' in the Old Testament.
The traditional way of reading this story in Hebrew is that 318 is the numerical value of the name of Abraham's servant Eliezer:
Jewish people have always had a system where each letter accorded with a specific numerical value. By adding up the six letters of the name in Hebrew, Jewish people learned that the author of Genesis secretly made reference to the fact that the 318 men was really a mystical reference to Moses's servant Eliezer. The difficulty with Barnabas, Clement and the rest of the Alexandrian Church was that they couldn't read Hebrew. They used a Greek translation of the Bible made long before the coming of Jesus were the same mystical reference to 318 was revealed.
It might amaze contemporary readers that the first Christians believed Jesus was present in the narrative. Yes, the Jesus who was commonly understood to be born to a virgin and fixed to a cross was also a magical presence in the Genesis narrative. But how did he get there? The Alexandrian tradition used a Greek translation of the Bible represents the number 318 with the Greek letters τιή or as our text of Barnabas now explains it:
For [the Scripture] says, And Abraham circumcised ten, and eight, and three hundred men of his household. What, then, was the knowledge given to him in this? Learn the eighteen first, and then the three hundred. The ten and the eight are thus denoted— Ten by Ι, and Eight by Η . You have Jesus. And because the cross was to express the grace by the letter Τ, he says also, Three Hundred. He signifies, therefore, Jesus by two letters, and the cross by one.Yet it is worth noting that when Clement was making reference to the same text - the Jesus reference was there but no equation of the T with the cross.
Did a fourth century editor manipulate the Letter of Barnabas in order to impose 'orthodox' notions of the appearance of the Cross? It is worth noting that the T shaped Cross only begins to appear in Christian art in the late fourth century. Clement by contrast only speaks of the number three hundred as being the 'Lord's sign' and other members of the Alexandrian tradition also know of the same mystery and make clear it has nothing to do with the shape of the cross. Perhaps most interesting of all is the fact that when Christian orthodoxy is established at the Emperor Constantine's summer retreat at Nicea exactly three hundred and eighteen attendees were present. The mystery of the Lord's sign being 'three hundred' being obscured for time immemorial.